Editing S-100 bus

{{short description|Early computer bus}}
{{Other uses|S-100 (disambiguation)}}
{{Infobox computer hardware bus
| name        = S-100 bus
| fullname    = 
| image       = Cromemco Blitz Bus.jpg
| alt         = 
| caption     = 
| invent-date = {{Start date and age|1974}}
| invent-name = [[Ed Roberts (computer engineer)|Ed Roberts]]
| super-name  = 
| super-date  = 
| replaces    = 
| width       = 8
| numdev      = 
| speed       = 
| style       = 
| hotplug     = 
| external    = 
}}

The '''S-100 bus''' or '''Altair bus''', later standardized as '''IEEE 696-1983''' ''(inactive-withdrawn)'', is an early [[computer bus]] designed in 1974 as a part of the [[Altair 8800]]. The {{nowrap|S-100}} bus was the first industry standard [[expansion bus]] for the microcomputer industry. {{nowrap|S-100}} computers, consisting of processor and peripheral cards, were produced by a number of manufacturers.  The {{nowrap|S-100}} bus formed the basis for homebrew computers whose builders (e.g., the [[Homebrew Computer Club]]) implemented drivers for [[CP/M]] and [[MP/M]].  These {{nowrap|S-100}} microcomputers ran the gamut from hobbyist toy to small business workstation and were common in early [[home computer]]s until the advent of the [[IBM Personal Computer|IBM PC]].

[[File:Harry Garland and Roger Melen, co-founders of Cromemco (1981).jpg|thumb|[[Harry Garland]] and [[Roger Melen]], co-founders of [[Cromemco]], holding an {{nowrap|S-100}} backplane (1981)]]

==Architecture==
The {{nowrap|S-100}} bus is a [[passive backplane]] of 100-pin printed circuit board edge connectors wired in parallel. Circuit cards measuring {{cvt|5|x|10|in|cm}} serving the functions of CPU, memory, or I/O interface plugged into these connectors. The bus signal definitions closely follow those of an 8080 microprocessor system, since the [[Intel 8080]] microprocessor was the first microprocessor hosted on the {{nowrap|S-100}} bus. The 100 lines of the {{nowrap|S-100}} bus can be grouped into four types: 1) Power, 2) Data, 3) Address, and 4) Clock and control.<ref name=Garland>{{cite book|last=Garland|first=Harry|title=Introduction to Microprocessor System Design|year=1979|pages=[https://archive.org/details/introductiontomi00garl/page/159 159–169]|quote=Although many other processors have been adapted to the {{nowrap|S-100}} bus, the bus signal definitions closely follow those of an 8080 system.|publisher=McGraw-Hill|location=New York |isbn=0-07-022871-X |url-access=registration|url=https://archive.org/details/introductiontomi00garl/page/159}}</ref>

Power supplied on the bus is bulk unregulated +8 Volt DC and ±16 Volt DC, designed to be [[Voltage regulator|regulated]] on the cards to +5 V (used by [[Transistor-transistor logic|TTL]] ICs), -5 V and +12 V for the [[Intel 8080]] CPU IC, ±12 V  [[RS-232]] line driver ICs, +12 V for disk drive motors. The onboard voltage regulation is typically performed by devices of the [[78xx]] family (for example, a 7805 device to produce +5 volts). These are [[linear regulator]]s which are commonly mounted on heat sinks.

The bi-directional 8-bit data bus of the Intel 8080 is split into two unidirectional 8-bit data buses. The processor could use only one of these at a time. The [[Sol-20]] used a variation that had only a single 8-bit bus and used the now-unused pins as signal grounds to reduce [[electronic noise]]. The direction of the bus, in or out, was signaled using the otherwise unused DBIN pin. This became universal in the {{nowrap|S-100}} market as well, making the second bus superfluous. Later, these two 8-bit buses would be combined to support a 16-bit data width for more advanced processors, using the Sol's system to signal the direction.

The address bus is 16-bits wide in the initial implementation and later extended to 24-bits wide. A bus control signal can put these lines in a [[Three-state logic|tri-state]] condition to allow direct memory access. The [[Cromemco Dazzler]], for example, is an early {{nowrap|S-100}} card that retrieved digital images from memory using direct memory access.

Clock and control signals are used to manage the traffic on the bus. For example, the ''DO Disable'' line will tristate the address lines during direct memory access. Unassigned lines of the original bus specification were later assigned to support more advanced processors. For example, the [[Zilog]] [[Zilog Z80|Z-80]] processor has a [[non-maskable interrupt]] line that the Intel 8080 processor does not. One unassigned line of the {{nowrap|S-100}} bus then was reassigned to support the non-maskable interrupt request.

==History==
[[File:Cromemco XXU S-100 processor.jpg|thumb|The Cromemco XXU processor board, introduced in 1986. At 16.7 MHz, it is the fastest CPU ever developed for the {{nowrap|S-100}} bus. It uses a Motorola 68020 processor with 68881 co-processor and 16 Kbytes of high-speed cache memory. This CPU is used in the Cromemco CS-250 computer, widely deployed by the U.S. Air Force.]] During the design of the Altair, the hardware required to make a usable machine was not available in time for the January 1975 launch date. The designer, [[Ed Roberts (computer engineer)|Ed Roberts]], also had the problem of the backplane taking up too much room. Attempting to avoid these problems, he placed the existing components in a case with additional "slots", so that the missing components could be plugged in later when they became available. The backplane is split into four separate cards, with the [[Central processing unit|CPU]] on a fifth. He then looked for an inexpensive source of connectors, and he came across a supply of military surplus 100-pin [[edge connector]]s. The 100-pin bus was created by an unknown draftsman at MITS, who selected the connector from a parts catalog and arbitrarily assigned [[signal (electrical engineering)|signal]] names to groups of connector pins.<ref>{{cite magazine| url=https://books.google.com/books?id=aj4EAAAAMBAJ&pg=PT54 |title=The {{nowrap|S-100}} Bus: Past, Present, and Future |magazine=InfoWorld |date=February 18, 1980 |pages=7,18 |first=Sol |last=Libes |volume=2 |issue=1}}</ref>

A burgeoning industry of "clone" machines followed the introduction of the Altair in 1975. Most of these used the same bus layout as the Altair, creating a new industry standard. These companies were forced to refer to the system as the "Altair bus", and wanted another name in order to avoid referring to their competitor when describing their own system. The "{{nowrap|S-100}}" name, short for "Standard 100", was coined by [[Harry Garland]] and [[Roger Melen]], co-founders of [[Cromemco]].<ref>{{cite book |last1=Freiberger |first1=Paul |author-link1=Paul Freiberger |last2=Swaine |first2=Michael | author-link2=Michael Swaine (technical author) |year=2000 |title=Fire in the Valley: The Making of the Personal Computer |url=https://archive.org/details/fireinvalleymaki00frei |url-access=registration |edition=Second | publisher=McGraw-Hill |isbn=0-07-135892-7 | page=[https://archive.org/details/fireinvalleymaki00frei/page/66 66]}}</ref><ref>{{cite journal|title=The Cromemco Story|journal=I/O News|date=September–October 1980|volume=1|issue=1|pages=10|url=https://archive.org/stream/IoNewsVolume1Number1/CromemcoIoNews11Sep-oct1980#page/n5/mode/2up|access-date=2013-02-22}}</ref> While on a flight to attend the Atlantic City PC '76 microcomputer conference in August 1976, they shared the cabin with Bob Marsh and [[Lee Felsenstein]] of [[Processor Technology]]. Melen went over to them to convince them to adopt the same name. He had a beer in his hand and when the plane hit a bump, Melen spilt some of the beer on Marsh. Marsh agreed to use the name, which Melen ascribes to him wanting to get Melen to leave with his beer.<ref>{{cite book |title= Fire in the Valley: The Birth and Death of the Personal Computer |first1= Michael |last1=Swaine |first2=Paul |last2=Freiberger |url=https://books.google.com/books?id=-g5QDwAAQBAJ&q=processor+technology#q=beer|isbn= 9781680503524 |date= 2014-10-20 }}</ref>

The term first appeared in print in a Cromemco advertisement in the November 1976 issue of [[Byte (magazine)|Byte magazine]].<ref name=herb>Herbert Johnson, [http://retrotechnology.com/herbs_stuff/s_origins.html "Origins of {{nowrap|S-100}} computers"], 15 March 2008</ref> The first symposium on the {{nowrap|S-100}} bus, moderated by [[Jim Warren (computer specialist)|Jim Warren]], was held November 20, 1976 at [[Diablo Valley College]] with a panel consisting of [[Harry Garland]], [[George Morrow (computers)|George Morrow]], and [[Lee Felsenstein]].<ref>
{{Cite journal
 | journal = Homebrew Computer Club Newsletter | title = Random Data | author = Robert Reiling
 | volume = 2
 | page = 1
 |issue = 11–12
 | date = December 10, 1976
 | url = http://www.digibarn.com/collections/newsletters/homebrew/V2_11-12/Homebrew_CC_Dec76_Page_01.jpg
 }}</ref> Just one year later, the {{nowrap|S-100}} Bus would be described as "the most used busing standard ever developed in the computer industry."<ref>{{cite book|last=Zaks|first=Rodnay|title= Microprocessors - From Chips to Systems| publisher=Sybex| year = 1977|page=302}}</ref>

[[Cromemco]] was the largest of the {{nowrap|S-100}} manufacturers, followed by [[Vector Graphic]] and [[North Star Computers]].<ref>{{cite journal|last=Libes|first=Sol|journal=Microsystems|date=September–October 1981|volume=2|issue=5|title=The leaders in the {{nowrap|S-100}} marketplace are Cromemco ($50M), Vector Graphics ($30M) and North Star ($25M)|page=8}}</ref> Other innovators were companies such as [[Alpha Microsystems]], [[IMS Associates, Inc.]], Godbout Electronics (later [[Bill Godbout|CompuPro]]), and [[Ithaca InterSystems]]. In May 1984, ''Microsystems'' published a comprehensive {{nowrap|S-100}} product directory listing over 500 "{{nowrap|S-100}}/IEEE-696" products from over 150 companies.<ref name="Sol">{{cite journal|last=Libes|first=Sol|title={{nowrap|S-100}} Product Directory|journal=Microsystems|date=May 1984|volume=5|issue=5|pages=59–78}}</ref>

The {{nowrap|S-100}} bus signals were simple to create using an 8080 CPU, but increasingly less so when using other processors like the 68000. More board space was occupied by signal conversion logic. Nonetheless by 1984, eleven different processors were hosted on the {{nowrap|S-100}} bus, from the 8-bit Intel 8080 to the 16-bit Zilog [[Zilog Z8000|Z-8000]].<ref name="Sol"/> In 1986, Cromemco introduced the XXU card, designed by Ed Lupin, utilizing a 32-bit [[Motorola]] [[68020]] processor.<ref>{{cite journal|title=New XXU Processor Offers Enormous Speed Advantage|journal=I/O News|date=August–September 1986|volume=5|issue=4|pages=1,9 | issn=0274-9998 |url=https://archive.org/details/IoNewsVolume5Number4/mode/1up}}</ref>

== IEEE-696 Standard ==

As the {{nowrap|S-100}} bus gained momentum, there was a need to develop a formal specification of the bus to help assure compatibility of products produced by different manufacturers. There was also a need to extend the bus so that it could support processors more capable than the Intel 8080 used in the original Altair Computer. In May 1978, [[George Morrow (computers)|George Morrow]] and [[Parasitic Engineering|Howard Fullmer]] published a "Proposed Standard for the {{nowrap|S-100}} Bus" noting that 150 vendors were already supplying products for the {{nowrap|S-100}} Bus. This proposed standard documented the 8-bit data path and 16-bit address path of the bus and stated that consideration was being given to extending the data path to 16 bits and the address path to 24 bits.<ref>{{cite journal|last1=Morrow|first1=George|last2=Fullmer|first2=Howard|title= Proposed Standard for the {{nowrap|S-100}} Bus |journal=Computer|publisher=IEEE Computer Society|quote= Extending the {{nowrap|S-100}} bus to 24 address bits and 16 data bits was recommended by Dave Gustavson. Exactly how this will be done is presently under consideration.
|date=May 1978|volume=11|issue=5|pages=84–90
|url= http://www.computer.org/csdl/mags/co/1978/05/01646965.pdf|doi=10.1109/c-m.1978.218190|s2cid=2023052}}</ref>

In July 1979 Kells Elmquist, Howard Fullmer, David Gustavson, and George Morrow published a "Standard Specification for {{nowrap|S-100}} Bus Interface Devices."<ref>{{cite journal|last1=Elmquist|first1=Kells A.
|last2=Fullmer|first2=Howard
|last3=Gustavson|first3=David B.
|last4=Morrow|first4=George |title= Standard Specification for S-100 Bus Interface Devices
|journal=Computer|publisher=IEEE Computer Society |date=July 1979|volume=12|issue=7|pages=28–52
|url= http://www.computer.org/csdl/mags/co/1979/07/01658813.pdf|doi=10.1109/mc.1979.1658813|s2cid=9797254
}}</ref>
In this specification the data path was extended to 16 bits and the address path was extended to 24 bits. The [[Institute of Electrical and Electronics Engineers|IEEE]] 696 Working Group, chaired by Mark Garetz, continued to develop the specification which was proposed as an IEEE Standard and approved by the [[IEEE Computer Society]] on June 10, 1982.<ref name="Standard">{{cite book| title=An American National Standard: IEEE 696 Standard Interface Devices
|doi=10.1109/IEEESTD.1983.81971|isbn=978-0-7381-4244-9}}</ref>

The [[American National Standards Institute]] (ANSI) approved the IEEE standard on September 8, 1983. The computer bus structure developed by Ed Roberts for the Altair 8800 computer had been extended, rigorously documented, and now designated as the American National Standard IEEE Std 696–1983.<ref name="Standard" />

== Retirement ==
[[File:Cromemco Z-2 Systems at Chicago Mercantile Exchange (1984).jpg|thumb|Racks of [[Cromemco]] {{nowrap|S-100}} Systems at the [[Chicago Mercantile Exchange]] in 1984]]

[[IBM]] introduced the [[IBM Personal Computer]] in 1981 and followed it with increasingly capable models: the XT in 1983 and the AT in 1984. The success of these computers, which used IBM's own, incompatible bus architecture, cut deeply into the market for {{nowrap|S-100}} bus products. In May 1984, Sol Libes (who had been a member of the IEEE-696 Working Group) wrote in [[Microsystems (magazine)|''Microsystems'']]: "there is no doubt that the S-100 market can now be considered a mature industry with only moderate growth potential, compared to the IBM PC-compatible market".<ref>{{cite journal|last=Libes|first=Sol|title=S-100 Product Directory|journal=Microsystems|date=May 1984|volume=5|issue=5|page=59|quote=However there is no doubt that the S-100 market can now be considered a mature industry with only moderate growth potential, compared to the IBM PC-compatible market.}}</ref>

As the IBM PC products captured the low-end of the market, {{nowrap|S-100}} machines moved up-scale to more powerful OEM and multiuser systems. Banks of {{nowrap|S-100}} bus computers were used, for example, to process the trades at the Chicago Mercantile Exchange; the United States Air Force deployed {{nowrap|S-100}} bus machines for their mission planning systems.<ref>{{cite journal|last=Breeding|first=Gary|title=Cromemco Systems Network Transactions at Chaotic Exchange|journal=I/O News |url=https://archive.org/details/io-news-volume-3-number-6/mode/2up| issn=0274-9998|date=January–February 1984|volume=3|issue=6|page=20}}</ref><ref>{{cite journal|journal=Aviation Week & Space Technology|title=USAF will equip its tactical fighter squadrons with a mission planning system | date=June 1, 1987|volume=126|issue=22|page=105}}</ref>  However throughout the 1980s the market for {{nowrap|S-100}} bus machines for the hobbyist, for personal use, and even for small business was on the decline.<ref>{{cite journal|last=Libes|first=Sol|title=S-100 Product Directory|journal=Microsystems|date=May 1984|volume=5|issue=5|page=59|quote=Whereas the early growth of the S-100 marketplace relied mainly on hobbyists and early personal computer users, the industry is now concentrating on OEM multiuser systems, and applications requiring more computer power.}}</ref>

The market for {{nowrap|S-100}} bus products continued to contract through the early 1990s, as IBM-compatible computers became more capable. In 1992, the Chicago Mercantile Exchange, for example, replaced their {{nowrap|S-100}} bus computers with the [[IBM Personal System/2|IBM model PS/2]].<ref>{{cite web|title=CME Taps Datacode To Distribute Quotation Data To Floor Traders|url=https://www.waterstechnology.com/data-management/1628751/cme-taps-datacode-to-distribute-quotation-data-to-floor-traders|publisher=WatersTechnology|date = January 27, 1992}}</ref> By 1994, the {{nowrap|S-100}} bus industry had contracted sufficiently that the IEEE did not see a need to continue supporting the IEEE-696 standard. The IEEE-696 standard was retired on June 14, 1994.<ref name="Standard" />

==References==
{{reflist|2}}

== External links ==
{{Commons category|S-100 bus}}
* [http://www.s100computers.com/index.html "S100 Computers"], A website containing many photos of cards, documentation, and history
* [http://www.kuhmann.com/Cromemco/CrosNest.htm ""Cromemco" based, S-100 micro-computer"] {{Webarchive|url=https://web.archive.org/web/20120210161228/http://www.kuhmann.com/Cromemco/CrosNest.htm |date=2012-02-10 }}, Robert Kuhmann's images of several {{nowrap|S-100}} cards
* [http://retrotechnology.com/herbs_stuff/s100.html "Herb's S-100 Stuff"], Herbert Johnson's collection of {{nowrap|S-100}} history
* [http://www.hartetechnologies.com/manuals "IEEE-696 / {{nowrap|S-100}} Bus Documentation and Manuals Archive"], Howard Harte's {{nowrap|S-100}} manuals collection

{{Computer-bus}}

[[Category:Computer buses]]
[[Category:Early microcomputers|*S-100]]
[[Category:IEEE standards]]
[[Category:Computer-related introductions in 1974]]
[[Category:Cromemco]]
[[Category:S-100 bus| ]]